This work will investigate: (a) subjective responses of normal humans to subanesthetic concentrations of nitrous oxide (N2O) in oxygen; and (b) the possible relationship between subjective N2O responses and subjects' antecedent drug abuse, personality, behavioral set, gas concentration (dose), and duration of inhalation. Results should give a comprehensive picture of subjective N2O effects, may provide data and methods for comparative psychoactive drug studies, and may contribute to knowledge of the psychopharmacology of abused drugs, more generally. Selected after extensive screening, four groups of equal N's, dichotomous both for extent of prior drug abuse and for scores on a personality dimension previously shown to be related to intensity of psychedelic drug responses, will receive further personality testing (measures related to perceptual sensitivity and "openness" to novel experiences), and will then inhale N2O six times under varied conditions of set, concentration, and duration. Gases will be administered by an anesthesiologist via a standard clinical gas anesthesia apparatus. Drug session experimenters will be blind both to gas concentrations and to personality test data. Three types of subjective response measures will be used. Relationships between independent variables and drug responses, including interaction effects, and individual differences in response patterns will be assessed.